An ingot can be cast by heating and melting a charge of material deposited in an open bottom electric induction cold crucible. Charge, for example in the form of raw or processed ore, can be fed into the crucible to maintain a molten mass (melt) of the material in the crucible as a portion of the molten mass solidifies and exits the bottom opening of the crucible as a formed ingot. The material must be electrically conductive in at least the molten (liquid) state for this electromagnetic casting process. Melting and heating of the charge can result in purification of the charge, for example, by impurities evaporating from the melt, or rising through the melt to float as dross at the surface of the melt within the crucible.
As mentioned above, the material does not necessarily need to be electrically conductive in the solid state. For example in a silicon electromagnetic casting process, room temperature solid non-electrically conductive silicon charge may be fed into the top of the crucible after a molten mass of electrically conductive silicon has been established within the crucible. U.S. Pat. No. 4,572,812 titled “Method and Apparatus for Casting Conductive and Semiconductive Materials” discloses a basic continuous silicon electromagnetic casting process, and is incorporated herein by reference in its entirety.
U.S. Pat. No. 4,572,812 (referred to as “the '812 patent”) discloses an electromagnetic casting process utilizing a single induction heating coil surrounding the exterior slotted wall (formed from a plurality of vertical members) of an open bottom electric induction cold crucible with the induction coil being connected at its terminals to a single RF power source.
The open bottom cold crucible may be installed in an enclosed chamber so that the heating, melting and/or solidification processes are accomplished in a vacuum or process gas environment. Further suitable cooling apparatus may thermally interact with the ingot as it exits the crucible so that the ingot's cooling rate over time is controlled until it reaches ambient temperature.
In other electromagnetic casting processes, two or more induction coils may be utilized in a stacked (adjacent) configuration around a partial exterior height of the crucible. For example, as shown in FIG. 1(a), FIG. 1(b) and FIG. 1(c), open bottom electric induction cold crucible 100 comprises a slotted wall formed from a plurality of vertical members 112 separated from each other by vertical slots 114 (shown as solid lines in the figures), with two separate induction coils 116a and 116b surrounding a partial exterior height of the crucible. The vertical slotted members are formed from a suitable material such as copper in this example, and may be connected at the top and bottom of the crucible. The connection between slotted members at the top of the crucible is almost always used and often provides the connection between each member and a water cooling circuit. The top connection is normally a significant distance from the melt and hence does not materially affect the induction coupling to the load of material in the crucible. The bottom connection, on the other hand, is not always used for smaller size crucibles but is more commonly used for larger crucibles where the connection provides support to the bottom of each vertical slotted member. In an electromagnetic casting process, the crucible slots are at least sufficiently long to support the inductive heating of the melt within the crucible and facilitate gradual cooling of the ingot as it is created at the solidification boundary 120 (as diagrammatically illustrated in FIG. 1(c)) until it exits the bottom of the crucible. It is the finding of this invention that, where the slots between vertical members do not extend to the bottom of the crucible (terminating at slot end 114a in FIG. 1 (c)), and hence a bottom copper (in this example) connecting member (horizontal) 117 is formed, the electromagnetic field generated by alternating current flow in induction coil 116b will tend to induce a circulating current which is very close to the load (ingot) as it exits the crucible. This proximity of the circulating currents generates heat in the load at a critical location where it may increase the risk of a run-out event that occurs when liquid silicon manages to find a way to the outer edge of the normally solidified edge of the cast ingot as it is drawn out of the crucible. The liquid silicon then flows in an uncontrolled manner into the bottom part of the furnace enclosure to cause damage to ancillary heaters, insulators and mechanical parts. Each of the coils may be connected to a separate alternating current (AC) power source operating at a different frequency. For example upper coil 116a may be operating at a frequency that is less than the frequency of the lower coil 116b. Flow of alternating current in each coil establishes a magnetic flux field that penetrates the slots (filled with an electrical insulating material) of the crucible to electromagnetically heat and melt an electrically conductive material placed within the interior crucible volume. As with all electric induction cold crucibles, the plurality of vertically members 112 making up the crucible's wall are cooled (typically by internally circulated water) so that the molten mass in contact with the wall freezes. This prevents contamination of the molten mass with wall material. The upper regions of the molten mass are at least partially supported by the Lorentz forces generated by the interaction of the magnetic field produced by the induction coils and the induced currents in the melt, to form a region of reduced contact pressure, or even separation, between the wall and the liquid mass of metal.
The advantage of multiple coils operating at different frequencies is the ability to lower the magnitude of the terminal voltage across each induction coil while still achieving a high level of induced energy transfer to the material within the crucible. This is of particular advantage when the electromagnetic casting process is performed with an oxidation prevention cover agent within the interior of the crucible that prevents oxidation of the molten material, as is the case in some silicon electromagnetic casting processes. Lower terminal voltages mitigate an arcing phenomenon between the melt and wall in the separation region mentioned above that can result in localized melting of the vertical members making up the crucible wall and migration of impurities from theses vertical members into the molten material within the crucible. The higher the terminal voltage across each coil the greater the risk of an arc. This is most significant when the interior cross sectional area of the crucible is sufficiently large to require a high coil terminal voltage to deliver sufficient induced energy to the melt in the crucible. In general when the interior cross sectional area of the crucible exceeds approximately 180 square inches, multiple coils operating at different frequencies are beneficial since this arrangement allows coil terminal voltages at less than 600 volts while an equivalent magnitude of induced energy can be transferred to the melt as would be done with a coil operating with a terminal voltage of 600 volts or more, and thus avoiding the melt contamination problem from arcing as described above.
The height of the open bottom electric induction cold crucible extends a distance, h1, below the lower end of lower induction coil 116b. Generally the vertical members 112 making up the wall of the crucible are sloped (tapered) outwards towards the open bottom of the crucible to facilitate movement of the formed ingot out of the crucible. In the two-coil arrangement shown in FIG. 1(a), the outward tapering may begin between the adjacent terminations of the upper and lower induction coils, to establish a taper distance of h2.
In some electromagnetic casting arrangements, an inter-coil magnetic shield 118 can be positioned between the adjacent ends of coils 116a and 116b to prevent mutual magnetic coupling (and interference) between the magnetic flux established by current flow in each of the two coils. Typical resulting magnetic flux patterns are represented by dashed lines in FIG. 1(a). Magnetic flux 116a′ is established by alternating current flow through upper coil 116a and magnetic flux 116b′ is established by alternating current flow through lower coil 116b. Magnetic flux field 116b′ extends below the bottom opening of the crucible. Such an arrangement results in anomalies around the outer perimeter of the formed ingot exiting the bottom of the crucible. The portion of the electromagnetic induction field which encompasses the bottom copper connecting member (horizontal) 117 of the crucible induces a circulating current which causes local heating of the surface of the load due to the fact that at the relatively high temperature the solid silicon is still partially conductive. This can cause a local change in the solidification temperature gradient which will increase stresses in the load and may increase the risk of run-outs (which would end the process and damage the equipment).
FIG. 1(d) illustrates the anomalies by a partial cross sectional thermal diagram near the bottom of the crucible. The dashed lines represent boundaries (contours) for typical temperature ranges in a silicon ingot being cast. The indicated range of numbers, for example, “20-19 kiloWatts (kW) per cubic meter (m3)” indicates a range of 20 to 19 kiloWatts per cubic meter of (volumetric) ohmic losses within the representative cross sectional contour in the silicon ingot being cast. Magnitude of ohmic losses within a region is representative of the temperature in the region. The adverse effect of local heating is demonstrated in the contours (region emphasized in single cross hatch) illustrating a relatively intense heating effect (ohmic loss) in the cast silicon adjacent to the bottom connecting member (horizontal) 117 and around that region a further contour of less intense heating (emphasized in double cross hatch).
It is one object of the present invention to eliminate the anomalies occurring around the bottom opening of an open bottom electric induction cold crucible used in an electromagnetic casting furnace that is caused by magnetic flux extension in the region of the bottom copper connecting member (horizontal) of the crucible.